This invention relates to a detecting device for detecting a metal strip embedded in paper money, such as a check or currency note in order to verify the authenticity of the check or currency note.
In recent years, the high quality of copying or printing machines has resulted in a serious counterfeiting problem. One of the countermeasures against counterfeiting which has been adopted in many countries is the use of a metal embedded in paper checks or currency notes. However, metal detectors for automatically discriminating between genuine and counterfeit currency notes and checks by detecting the metal strip have not been put to practical use because they have not been capable of accurate and reliable detection of the metal strips at high speed.
One known technique for detecting the metal strip embedded in paper money is disclosed in United Kingdom Patent Appication No. 2,017,996. FIG. 1(a) of the drawings of the present application illustrates the detecting system of this U.K. patent application. The detecting system includes a circuit formed by resistor R, self-induction coil L and adjustable variable capacitor C. A voltage U.sub.1 having a frequency of 2 MHz is applied to this circuit and an output voltage U.sub.2 is measured at the terminals of capacitor C. The high frequency output voltage U.sub.2 is modulated by movement of paper money P, metal strip M in front of the coil L in the direction of the arrow. The metal strip M crosses the lines of magnetic induction generated by coil L to modulate the output voltage U.sub.2. However, because of coil L is positioned on only one side of paper P, undesirable variations in the output voltage U.sub.2 occur. In other words, a little variation in the space S between coil L and paper money P varies the number of lines of magnetic induction crossed by metal strip M which causes a noticeable variation in output voltage U.sub.2.
The graph in FIG. 1(b) shows the variation of output voltage U.sub.2 as a function of the variation in the space S between paper money P and coil L. As apparent from this graph, the variation .DELTA.R in output voltage U.sub.2 is large compared to the space variation .DELTA.S between paper money P and coil L. Since paper money P, such as a currency note, is transported at high speed, the space between the detecting coil and the currency note cannot be maintained constant. As a result, frequent detection errors occur in this prior art detecting system.